1. Field of the Invention
The present invention relates to a mask for photolithography having a plurality of transmission areas, a method of forming a thin film using the mask for photolithography, a liquid crystal display device having a thin-film pattern comprising portions with different thicknesses formed on a glass substrate by a single process, and a method of producing the liquid crystal display device.
2. Description of the Related Art
An increase in the size of a screen of a liquid crystal display device, such as a liquid crystal display, and an increase in the efficiency of a production process have caused a photolithography process for forming a thin-film pattern onto a glass substrate used in the liquid crystal display device to become a large-scale process. In carrying out such a large-scale photolithography process, a large photo-mask, which allows a large substrate material to be efficiently exposed, is used because exposure using a stepper results in poor production efficiency.
For example, diversification of liquid crystal modes has caused a demand for a multi-step film structure that is formed by carrying out the photolithography process a plurality of times. However, an increase in the number of times the photolithography process is carried out increases the process time. This applies to, for example, the case where color filters are formed with multiple steps in forming reflective-and-transmissive liquid crystals, and the case where a thin-film pattern with multiple steps is formed on a glass substrate for, for example, providing multiple gaps. This increase in the number of times the photolithography process is carried out is an obstacle to reducing costs and lead time.
A method using a halftone mask having a plurality of transmission areas of different light transmittances formed on one mask substrate is one means for solving the above-described problems. In this method, by forming transmission areas of different light transmittances in a photo-mask, the amount of exposure light on each portion of a substrate (hereinafter referred to as “the target substrate”) onto which a thin-film pattern is formed is adjusted, so that a thin-film pattern comprising portions with a plurality of thicknesses is formed by carrying out the photolithography process once. This method makes it possible to form a thin-film pattern having a multi-step structure by carrying out the photolithography process once because a photosensitive material on the target substrate is exposed to exposure light of different intensities.
However, when the above-described halftone mask is used, interference between diffracted lights occurs at the boundaries of the transmission areas having different light transmittances. When the diffracted lights interfere to weaken each other, film-diminishing steps are formed in the thin-film pattern formed on the target substrate because there are portions thereof exposed to weaker light. For this reason, when the halftone photo-mask is used, it is difficult to properly form a thin-film pattern having a multi-step structure by a single process due to the effects of interference between the diffracted lights mentioned above.
In order to eliminate the effects of diffracted lights, it is desirable to carry out the photolithography process by bringing the photo-mask and the target task as close to each other as possible. However, when the photo-mask and the target substrate contact each other due to the effects of, for example, flexing of the photo-mask or the waviness of the target substrate, contamination or scratching of the photo-mask, improper film deposition, or the like, occurs. In particular, a photo-mask needs to be replaced every time contamination occur, thereby reducing production efficiency and increasing costs, so that it is necessary to perform exposure with a predetermined gap between the photo-mask and the target substrate. In addition, when the size of the substrate material, flexing of the photo-mask and the waviness of the target substrate tend to increase, so that it is necessary to make the gap between the photo-mask and the target substrate even larger. Combined with an increase in the fineness of a mask pattern, this increases the effects of diffracted lights on the thin-film pattern on the target substrate.